Chang'e-6 lunar mission finds impact-created hematite and maghemite in SPA Basin samples

Redox reactions are fundamental to the history of planetary formation, yet previous studies confirmed that neither the Moon's interior oxygen fugacity nor its surface environment favor strong oxidation. Most iron on the Moon is found in reduced forms, either as ferrous or metallic iron. However, spectroscopic data recently suggested that hematite is distributed in high-latitude lunar regions.

Earlier investigations into Chang'e-5 samples discovered sub-micron magnetite and signs of Fe3+ in impact glasses. Such results point to localized oxidizing environments formed during impact-modification of the surface. Until now, however, mineralogical proof of strongly oxidizing minerals like hematite on the Moon had been unavailable, and debate persisted over the extent and presence of such minerals.

The SPA Basin, among the Solar System's largest and oldest impact sites, offers rich ground for investigating lunar oxidation. The Chang'e-6 mission's sample return from this basin enabled the discovery of micron-scale hematite grains. The team used micro-area electron microscopy, electron energy loss spectroscopy, and Raman spectroscopy to confirm both the crystal structure and lunar origin of these minerals, ruling out terrestrial contamination.

The study concludes that hematite formed during ancient large impact events, where the immense heat vaporized surface material and briefly produced a high-oxygen-fugacity environment. Released iron ions were oxidized and deposited as micron-sized hematite through vapor-phase processes. Hematite was found alongside magnetic magnetite and maghemite.

Persistent magnetic anomalies on the Moon, including areas in the SPA Basin, have lacked clear explanation. Because oxidation influences the development of magnetic carrier minerals, these findings deliver important evidence for the history and nature of lunar magnetic features.

The research challenges the long-held assumption that the lunar surface is uniformly reduced, providing valuable data for understanding the evolution of lunar magnetic anomalies and the mechanisms of large impact events.